The Obama administration on Tuesday was set to build the public case for a military strike on Syria by releasing documents on last week's alleged chemical weapons attack -- with one senior Defense official telling Fox News that a U.S. strike is "not a matter of if, but when."

One day after Secretary of State John Kerry declared that the evidence of a chemical weapons attack last week in Syria in "undeniable," the White House was expected to release intelligence backing up that claim. A source close to the White House told Fox News that the documents will probably be released on Tuesday.

Kerry and others have said the U.S. and its allies must respond, if for no other reason than to deter governments around the world from using chemical weapons in the future. Opposition and human rights groups in Syria have said the attack last week killed hundreds, though estimates of the death toll fluctuate widely.

In Washington, few in the administration doubt that military action will occur. U.S. military sources tell Fox News that the planned military operation would include cruise missiles and possible long-range stealth bombers but will not target Syria's chemical weapons stockpiles.

"There is no way to 'surgically strike' Syria's chemical weapons stockpiles" without boots on the ground, a source said.

Meanwhile, NBC News reports that missile strikes could be launched as early as Thursday.
Senior U.S. Defense officials tell Fox News that military assets are in position and could strike at any moment, once President Obama gives the order.

The administration, though, has decided to work through NATO; an emergency session in Brussels has been called for Wednesday. Any military action is unlikely to take place before that meeting.

Also, British Prime Minister David Cameron has scheduled a vote for Thursday in the British Parliament, calling them back from recess. It is also unlikely that any military strike would occur before that vote, as the Obama administration wants to have British support.

U.S. officials tell Fox News that no strike is likely while U.N. inspectors are still on the ground in Damascus, either.